The interconnection of a number of electrical equipment for supervisory control and data acquisition is described within U.S. patent application Ser. No. 08/628,533 filed Apr. 3, 1996 entitled "Dynamic Data Exchange Server Simulator". The electrical equipment is interconnected within a LAN utilizing a Modbus protocol which is a trademark of New Modicon Inc. for RS-485 data format.
The Modbus protocol calls for a master-slave regime for communicating with the PC or CPU, as master, on the communicationis bus and the associated electrical equipment or devices, as slaves. The master is able to communicate with any of the associated devices which are then only able to respond and communicate with the master. A good definition of the master-slave regime is found in U.S. Pat. No. 4,817,037 entitled "Data Processing System With Overlap Bus Cycle Operations".
With such master-slave regimes, in the event the master is unable to period the corresponding slave electrical devices would operate independently without instructions from the master. Alternatively, the communications bus could be disabled while removal and repair of the master is undertaken.
U.S. patent application Ser. No. 08/804,052 filed on Feb. 21, 1997 entitled "Interface Monitor for Communicating Between Different Communications Protocols" describes a supplemental monitor unit that connects with the communication bus as well as with a PC, acting temporarily as a master, to allow the PC to interrogate the associated electrical devices without hardware connection between the PC and the communication bus.
It has been determined that the same monitor unit can also be used to act as a temporary master, i.e. provide "fault tolerance" in the event that the system's master becomes unable to communicate.
One purpose of the instant invention, is to provide a monitor unit that allows communication with electrical elements or slaves interconnected with a communication bus when the PC or master is no longer capable of transmission.
In some industrial applications, the master is remote from the associated electrical devices and the monitor is more readily accessible by an on-site operator. The monitor is then interrogated to obtain data from any of the electrical devices and the master addresses the electrical devices allow the monitor to obtain the data from the LAN. In accordance with the master-slave protocol the monitor, as a slave, is unable to directly interrogate the master to obtain the data.
Earlier attempts to improve this problem incur the use of additional multiplexing equipment for switching between the master and the monitor for communicating on the LAN such as proposed by the Square D Power Logic Network type 8030 CRM 565. A later approach to this problem is the incorporation of a pass-through module such as the Cutler Hammer type AEM11.
It would be advantageous as to time and expense to allow a monitor to obtain device data from any of the devices in real time.
A further purpose of the invention, is to provide an operating program within the master that allows the monitor to obtain data from any of the devices without incurring additional equipment and without having to incur a measurable time delay.